Fetal heart action is commonly monitored during labor and delivery. Dramatic changes in fetal ECG measurements are generally indicative of fetal distress. A variety of ECG probes have been designed which are inserted through the vagina and cervix of the mother and attached to the epidermis of the fetus. Only physical contact between an electrode and the fetus is necessary to obtain good ECG readings. Typical ECG probes include an electrically conductive spiral needle which is subcutaneously introduced into the fetal scalp at one end and electrically connected to an amplifier and a cardiotachometer (i.e., an electrocardiograph) at the other end for measuring and recording the electrical impulses which initiate contraction of the heart.
Several such ECG probes are described in U.S. Pat. No. 4,244,375 (issued to Farrar et al.), U.S. Pat. No. 4,321,931 (issued to Hon) and U.S. Pat. No. 5,150,709 (issued to Neward). Such probes are well known and widely used in the medical profession.
Fetal blood pH is also commonly monitored during labor and delivery. Fetal blood pH is a well known measure of the metabolic and respiratory status of a fetus. The pH of fetal blood is dependent in large measure upon the concentration of carbon dioxide and acid in the blood, with an increase in carbon dioxide and/or acid producing a corresponding decrease in pH. A decrease in pH values suggests that the concentration of carbon dioxide in the fetal blood has increased, which is an early warning sign that the fetus is receiving insufficient oxygen. Monitoring of fetal blood pH is particularly useful for corroborating a diagnosis of fetal hypoxia based upon irregular fetal heart rate patterns.
A first type of pH probe utilizes a standard electrode cell assembly which includes a measuring electrode and a reference electrode. The construction of such pH probes is well known in the industry. The probe is subcutaneously introduced into the fetal scalp with the measuring electrode in contact with the biological fluid to be measured (generally blood). The measuring and reference electrodes are electrically connected to an amplifier and a recording device for measuring the electrical potential measured by the electrodes and recording those values as pH.
Several such electrode cell pH probes are described in U.S. Pat. No. 3,224,433 (issued to Von Dalebor), U.S. Pat. No. 3,959,107 (issued to Horner et al.), U.S. Pat. No. 3,973,555 (issued to Moller et al.), and U.S. Pat. No. 4,281,659 (issued to Farrar et al.). Such probes are well known and widely used in the medical profession.
A second type of pH probe utilizes optical fibers and a pH sensitive dye composition to measure pH. A first embodiment of such optical pH probes includes a pair of optical fibers are retained within an ion permeable envelope. A pH sensitive dye (i.e., a dye whose color intensity changes with changing pH values) is provided within the envelope. One of the optical fibers is connected to a light source for transmitting light into the envelope while the second optical fiber is connected to a light sensor for measuring the amount of light transmitted from the first optical fiber to the second optical fiber after passing through the pH sensitive dye composition. The measured light intensity can be directly correlated to pH based upon the known relationship between the color intensity of the pH sensitive dye composition and pH.
A second embodiment of such optical pH probes includes a single optical fiber containing a pH sensitive fluorescent dye in the distal end of the fiber. The proximal end of the optical fibers is connected to a light source for transmitting light of a known intensity down the fiber and into contact with the pH sensitive dye. The proximal end of the fiber is also attached to a light sensor for measuring the amount of light which is transmitted back up the optical fiber by the dye. The measured light intensity is dependent upon the extent to which the dye is able to absorb and fluoresce, a characteristic which can be directly correlated to pH based upon the known relationship between the fluorescent capacity of the dye and pH.
A fiber optic pH probe of the latter type is manufactured and sold commercially by several suppliers, including Ensign-Bickford Optics Company of Avon Connecticut.
Several attempts have been made to combine an ECG probe and a pH probe in a single device. Examples of such efforts are disclosed in U.S. Pat. No. 4,294,258 (issued to Bernard), U.S. Pat. No. 4,658,825 (issued to Hochberg et al.), and U.S. Pat. No. 4,320,764 (issued to Hon). Exemplary of these combination devices, the device disclosed by Hochberg et al. incorporates a fiber optic pH probe within the lumen of a typical electrically conductive spiral needle so that both the ECG probe (the needle) and the pH probe are subcutaneously positioned into contact with fetal interstitial tissue and fluids with a single puncture site for continuous in-vivo monitoring of ECG and fetal pH.
While the combination devices disclosed by Bernard, Hochberg et al. and Hon constitute a significant advance over the rather cumbersome and invasive technique of inserting and attaching multiple devices in order to monitor heart rate and pH, they have not been widely used within the medical field as they tend to suffer from a gradual loss of accuracy in pH measurement. It is believed that such loss of accuracy is due to the static nature of the probes relative to fetal tissue and fluids and the gradual dampening of normal biological ionic interchange at the incision cite.
Hence, research continues in an effort to develop a fetal ECG and pH monitoring device which is simple to use, reliable and provides accurate pH readings over extended periods of time.